1. Field of the Invention
This invention relates to head stack assemblies and to magnetic disk drives. In particular, the present invention relates to head stack assemblies and disk drives including a pivot-bearing cartridge including a damping layer to reduce vibrations in the actuator assembly.
2. Description of the Prior Art and Related Information
As the storage capacity of disk drives increases, the number of tracks per inch (TPI) on which data is stored also increases. As the TPI increases, the importance of the servo bandwidth controlling the head correspondingly increases. As the actuator is controlled to move ever faster, unwanted resonances occur that are detrimental to the operation of the drive. Such actuator resonances transfer to the suspension assembly to which the read/write heads are located, to the pivot-bearing cartridge, as well as to the base and the cover of the drive. Theses vibrations contribute to off-track motion of the read/write heads as they follow the data tracks on the recording surface of the magnetic disk during reading and writing operation.
Although disk drive designers strive to render the actuator assembly as stiff as possible, it is still subject to bending when forces incident to the normal operation of the drive act upon it. In particular, the actuator assembly may exhibit undesirable resonances, such as the so-called butterfly mode resonance (also called the main mechanical system mode or the first principal bending mode of actuator), in which the actuator elastically bends about the pivot-bearing cartridge. In current designs, such butterfly mode resonances may be centered at about 6 Khz. Efforts in reducing the effect of such butterfly mode resonance have centered on increasing the frequency of the butterfly mode resonance by increasing the stiffness of the actuator assembly and decreasing its mass. However, even though the butterfly mode frequency increases, the amplitude of the corresponding resonance peak, as measured on a Bode plot, may still be unacceptably high. Designers have also used a powerful servo notch filters to filter out the butterfly mode resonance frequencies. Another alternative is to use a less powerful servo notch filter and to reduce the amplitude of the butterfly resonance peak by introducing damping into the system. Using damping and a less powerful servo notch filter, however, does not decrease the servo bandwidth as much as using a powerful servo notch filer without damping.
U.S. Pat. No. 6,411,472 discloses a disk drive in which a tolerance ring is interposed between the through bore defined within the head stack assembly and the pivot-bearing cartridge. The tolerance ring in this patent has a laminate structure in which two metal layers sandwich a viscoelastic dampening layer. However, it is believed that manufacturing such a dual metal laminate tolerance is difficult, impractical and expensive. Indeed, during manufacture of the rings, the sheet metal that forms the dual laminate structure is pressed against gears to form the indentations that impart the required cylindrical shape to the tolerance ring. So that the locations of the indentations precisely match on both sheets, the two sheets of metal must be pressed against the gears at the same time. This, however, tends to delaminate the dual metal laminate structure of the tolerance ring. If, however, the sheets are not pressed against the gears at the same time, the resulting indentations on both sheets likely will be offset relative to one another, further promoting delamination of the laminate structure or further difficulties in laminating the dual metal laminate tolerance ring in the first place.
What are needed, therefore, are disk drives and head stack assemblies that incorporate damping to reduce the deleterious effects of unwanted resonances. Such disk drives and head stack assemblies should incorporate such damping in an easily and practically manufacturable manner, and in a manner that is relatively inexpensive.